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act to open membrane channels permeable to Na + , leading to a Na + influx and membrane
depolarization (Collingridge and Bliss, 1987). Only the channel opened by NMDA receptor
accessible to kainate, quisqualate and to domoic acid are, in addition, highly permeable to Ca 2+
and induced lethal cellular Ca 2+ entry.
7.3.2.8 Tolerance levels and remarks on safety
An effect on certain consumers of domoic acid contaminated shellfish was inferred at
an estimated concentration of 200 µg.g -1 wet wt. So an application factor of 0.1 was applied for
safety, and a concentration of 20 µg.g -1 wet wt was set as the level of domoic acid above which
a shellfish operation should be closed (Waldichuk, 1989). This compares with 0.8 µ.g -1 for
saxitoxin in shellfish, above which an area is closed for shellfish harvesting owing to PSP. With
regard to health safety, a concentration of 20 µg/g of domoic acid in a fresh weight of molluscs
is considered as tolerable (Waldichuk, 1989).
Fig. 24 Postsynaptic receptors of neurotransmitter amino acids and channels of sodium and
calcium.
N = NMDA receptor
KQ = Kainate and quisqualate receptors